1. Field of the Invention
The present invention relates to an LED-switching controller employing a booster for controlling switching parallel-connected LEDs
2. Description of the Related Art
Recent various electronic devices including cellular phones are provided with a plurality of LEDs for indication and decoration. Cellular phones are subject to restrictions on the capacity of the power supply and most cellular phones are provided with a boosting circuit for switching on the LEDs. A prior art boosting circuit for switching a plurality of parallel-connected LEDs disclosed in U.S. Pat. No. 6,538,394 measures the voltage applied to a signal line connected to any one of the LEDs, and raises the measured voltage to a reference voltage for driving the LED in a current control mode. The reference voltage is determined taking into consideration the different forward voltages necessary for driving the plurality of individual LEDs in a current control mode. The reference voltage thus determined is often an excessively high voltage.
White LEDs have greatly different characteristics, respectively, and the LEDs cannot be driven so as to shine in luminances in a predetermined luminance range unless a voltage necessary for driving all the LEDs is applied to the LEDs. Therefore, even if a voltage necessary for driving some LED by a constant current is created by the boosting circuit, the voltage is not necessarily a voltage sufficient for driving other LEDs by a constant current, and it is possible that the respective luminances of the plurality of LEDs are distributed in a range wider than an allowable range or some of the plurality of LEDs cannot be switched on. Thus, a comparatively high voltage must be applied to the LEDs to drive the LEDs in a current control mode to avoid the foregoing trouble.
Thus, a plurality of LEDs cannot be driven so that all the LEDs shine in luminances in a predetermined luminance range by using a single boosting circuit. If an excessively high voltage is used for driving the LEDs in a current control mode the efficiency of the boosting circuit decreases.